The Transportation Department said airlines have long known that as part of rules governing fare advertising they must have a "reasonable number" of seats available at the advertised price... The department said it checked and found that Southwest had no seats for $59 to those three cities on any of the travel dates listed in the ad.

The Department of Transportation hit Southwest with a $200,000 fine and told them not to do it again.

DOT's Aviation Enforcement Office found that Southwest advertised one-way, nonstop fares "for $100 or less" for travel on Feb. 14, 2013, but failed to include a reasonable number of seats available in a significant number of city-pair markets in the fare sale. In addition, on Jan. 30, 2013, Southwest advertised $66 one-way fares from Dallas Love Field to Branson, Mo., between March 1, 2013, and March 21, 2013. However, there were no seats available at the sale fare on any day during the sale period.

Southwest told the AP it was all a big misunderstanding due to "a flawed review of the ad copy."

They did say that although the seats were never actually available at the advertised rate, the company honored the promotion for customers who called and demanded it.